Note: Descriptions are shown in the official language in which they were submitted.
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TITLE OF THE INVENTION
METHOD FOR IDENTIFYING MODULATORS OF THE NRF2-KEAP I -ARE PATHWAY
BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates to a method for identifying modulators of the
Keapl-Nrf2-ARE pathway. In particular, the present invention relates to an
assay for identifying
molecules that inhibit binding of a labeled Nrf2 peptide with the kelch domain
of the Keapl
protein. Molecules that inhibit binding of the labeled peptide to the Keapl
protein can be
activators of the Keapl-Nrf2-ARE pathway. Activation of the Keapl-Nrf2-ARE
pathway results
in an increased accumulation of Nrf2 and the subsequent induction of
protective enzymes, for
example, the phase 2 detoxification enzymes. Activators of the Keapl-Nrf2-ARE
pathway are
useful for combating oxidative stress=related disorders, such as those
associated with cancer,
emphysema, Huntington's disease, light-induced retinal damage, and stroke.
(2) Description of Related Art
Oxidative stress is a well-studied, but poorly controlled, component of
cellular
toxicity in which highly reactive molecules damage DNA, proteins, and lipids.
An imbalance
between prooxidant species (including superoxide anion, peroxynitrite, and the
hydroxyl radical)
and the body's antioxidant defenses can lead to disruption of cellular
function and may
contribute to disease initiation and progression. Not all free radical
production is deleterious to
the body, however. Myeloperoxidase catalyzes the production of hypochlorous
acid from
hydrogen peroxide and chloride anion during the neutrophil's respiratory burst
(Klebanoff, J.
Leukoc. Biol. 2005, 77:598-625). This rapid, localized release of reactive
oxygen species is
critical to humans in the protection from bacteria; myeloperoxidase deficiency
may result in a
compromised immune response or higher incidence of cancer (Lanza, J. Mol. Med.
1998,
76:676-68 1). Thus, free radical production and quenching must be closely
regulated so as to be
permissive of desirable reactive oxygen species functions without allowing
excessive free radical
accumulation. It has been proposed that the cell accomplishes this feat
through the
compartmentalization of free radicals into "microdomains" (Terada, J. Cell
Biol. 2006, 174:615-
623), which would suggest that the cell has evolved a cytoplasmic redox
sensing mechanism to
identify and trigger an antioxidant response to quench the excessive
accumulation or release of
prooxidant species from their restricted locations.
Exogenous antioxidant therapies have been proposed to restore the redox
balance
of the cell. Unfortunately, clinical efforts utilizing exogenous antioxidant
therapies have, to date,
generated only modest or ambiguous results. These results would indicate the
complexity of
exogenous antioxidants as therapeutics, and may indicate the need to utilize a
more refined
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method of combating oxidative stress. Although a complete listing of clinical
trials involving
antioxidant therapies is beyond the scope of this description of the related
art, an examination of
clinical trials involving alpha-tocopherol demonstrates the disappointing
results achieved with
exogenous antioxidant therapy. In reviews of completed clinical trials
evaluating alpha-
tocopherol therapy in cardiovascular disease, cancer, Parkinson's,
Alzheimer's, tardive
dyskinesia, and cataract, the available data did not justify a recommendation
for use of the
antioxidants for disease prevention, with the potential exception of alpha-
tocopherol
supplementation for patients with recently diagnosed tardive dyskinesia
(trials reviewed in Pham
et al., Ann. Pharmacother. 2005, 39:1870-1878; Pham et al., Ann. Pharmacother.
2005, 39:2065-
2072). In fact, the use of some exogenous antioxidant therapies has been shown
to worsen
outcome in certain patient populations. In a clinical trial involving 20 mg
per day beta-carotene
supplementation, the researchers observed a higher incidence of lung cancer
among those
receiving beta-carotene compared to those who did not (The Alpha-Tocopherol,
Beta Carotene
Cancer Prevention Study Group. N. Engl. J. Med.1994, 330:1029=1035). It has
been proposed
that clinical efforts involving high doses of antioxidants may interfere with
cell functions
involving redox signaling. A recent report of a clinical trial involving long-
term dosing of a
cocktail of antioxidants given at low, nutritionally-available doses
demonstrates a decreased risk
of cancer in males, but not in females (Hercberg et al., Arch. Intern. Med
2004, 164:2335-2342).
It has been proposed that this gender bias is due to the lower basal levels of
endogenous
antioxidants in the male cohort when compared to the females (Galan et al.,
Br. J. Nutr. 2005,
94:125-132). These various clinical efforts suggest that the "tone", or amount
of antioxidant
(whether exogenous or endogenous), along with an understanding of the
pharmacokinetic
parameters of the antioxidant used will be critical to the success of any
therapeutic.
The antioxidant response element (ARE) is a cis-acting regulatory element
found
in the 5'-flanking region of genes encoding a number of cytoprotective enzymes
and regulates the
expression of these proteins in response to oxidative stress (Rushmore et al.,
J. Biol. Chem.
1991, 266:11632-11639). The coordinate upregulation of these genes results
from the
phosphorylation and translocation of the transcription factor Nuclear Factor
Erythroid 2-like 2
(Nrf2) to the nucleus in response to stress on the cell. Once in the nucleus,
Nrf2 forms a
complex with small musculoaponeurotic fibrosarcoma oncogene (Maf) proteins and
other
components of the transcriptional machinery to induce expression of ARE-
containing promoters
(Itoh et al., Mol Cell Biol 1995, 15:4184-4193; Itoh et al., Biochem Biophys
Res Commun 1997,
236:313-322; Moi et al., Proc. Natl. Acad. Sci. U S A 1994, 91:9926-9930).
Under basal
conditions, Nrf2 is bound to Keapl, a cysteine rich E3 ubiquitin ligase
substrate adaptor protein
that is part of the ubiquitin-proteosome degradation pathway.
The Keapl protein is comprised of several distinct domains; an N-terminal
region
of 66 amino acids, a BTB domain from amino acid residue 67 to 178, a 137 amino
acid BACK
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domain (Stogios and Prive, Trends Biochem. Sci. 2004, 29:634-637) also known
as the linker,
central linker domain, or intervening region (IVR), a kelch domain comprised
of amino acid
residues 322 to 608, and a C-terminal region of 15 amino acids. Systematic
deletion of each of
these domains reveals that the kelch domain is required to bind and sequester
Nrf2 and that the
BTB and BACK domains are required for the modulation of Nrf2 levels by
chemical agents
(Zhang and Hannink, Mol. Cell. Biol. 2003, 23:8137-8151). In addition, the BTB
domain of
Keapl is critical for dimerization of Keapl (Zipper and Mulcah, J. Biol. Chem.
2002,
277:36544-36552). Several groups have provided convincing evidence
demonstrating that
Keap 1 acts as a substrate-specific adaptor in an E3 ubiquitin ligase complex
that ubiquitinates
and targets Nrf2 for degradation by the proteosome (Furukawa and Xiong, Mol.
Cell. Biol. 2005,
25:162-171; Kobayashi et al., Mol. Cell. Biol. 2004, 24:7130-7139; Zhang et
al., Mol. Cell. Biol.
2004, 24:10941-10953). The ubiquitin-proteosome system comprises one of
nature's most oft-
repeated means of regulating protein levels within the cell. Three component
enzymes, an E1
ubiquitin-activating enzyme, an E2 ubiquitin-conjugating enzyme, and an E3
substrate adaptor
protein complex work in conjunction to covalently attach ubiquitin to a
substrate. Subsequent to
the first ubiquitin addition, a polyubiquitin chain may be synthesized. These
ubiquitinated
substrates are then targeted for degradation by the 26S proteosome. Cu13
(Cullin 3) is a core
scaffolding protein in the E3 ligase complex and has direct protein:protein
interactions with both
Keap 1 and Rbx 1(Ring box 1). Cu13 and Rbx 1 form the catalytic component of
the enzyme
complex and interact with an E2 ubiquitin ligase to transfer ubiquitin to the
substrate.
Under oxidative load, Nrf2's degradation by Keap 1 is disrupted, resulting in
the
nuclear accumulation of the transcription factor and enhanced transcription
(Dinkova-Kostova et
al., Proc. Natl. Acad. Sci. U S A 2002, 99:11908-11913; Itoh et al., Genes
Dev. 1999, 13:76-86;
McMahon et al., J Biol Chem 2003, 278:21592-21600; Zhang et al., Mol Cell
Biol. 2003,
23:8137-8151). Several efforts to define the full gene list regulated by ARE
enhancer elements
have been made. These include comparative profiling of small molecules
inducers of the
pathway in wild-type and Nrf2 (-/-) mouse, such as the isothiocyanates phenyl
isothiocyanate
(PEITC) (Hu et al., Life Sci. 2006, 79:1944-1955) and sulforaphane (Hu et al.,
Cancer Lett 2006,
243:170-192; Thimmulappa et al., Cancer Res 2002, 62:5196-5203) as well as the
green tea
extract (-)-epigallocatechin-3-gallate (EGCG) (Shen et al., Pharm. Res. 2005,
22:1805-1820) and
3H-1,2-dithiole-3-thione (D3T) (Kwak et al., J. Biol. Chem. 2003, 278:8135-
8145). Consensus
genes and gene families commonly induced in these analyses include ferritin,
heme-oxygenease 1
(HO-1), NAD(P)H:quinine oxidoreductases (NQO1), glutamate cysteine ligase
catalytic (GCLC)
and glutamate cysteine ligase modifier (GCLM) subunits, and glutathione-S-
transferases (GSTs).
Disruption of the interaction between Keapl and Nrf2 results in activation of
the
Keap 1-Nrf2-ARE pathway. The genetic knockdown of the Keap 1 protein in human
keratinocyte
cell line provides evidence that relief of Nrf2 repression results in
transcription of ARE-
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dependent genes (Kwak et al. ibid.). In these studies, a 70% reduction in
Keapl mRNA and a
corresponding reduction in Keap1 protein levels followed transfection of anti-
Keap1 siRNA.
Within 24 hours of transfection, Nrf2 protein levels and transcription of an
ARE-luciferase
reporter gene construct were increased. A 3-fold increase in NQO1, 5-fold
increase in GCLC,
and a 2.5 fold increase in GCLM within this cell line verify that disruption
of Keap 1's capacity to
regulate Nrf2 protein levels result in increases in ARE-dependent gene
transcription (Kwak et al.
ibid).
Evidence has emerged over the last decade indicating that activation of the
antioxidant response element may be beneficial to the whole organism. Nrf2
mediated activation
and resultant ARE-regulated gene induction results in improved outcomes in
several animal
models of disease, including cancer (Iida et al., Cancer Res. 2004, 64:6424-
6431; Ramos-Gomez
et al., Proc. Natl.Acad. Sci. U S A 2001, 98:3410-3415; Xu et al., Cancer Res.
2006, 66:8293-
8296; Yates et al., Cancer Res. 2006, 66:2488-2494), Huntington's (Shih et
al., J. Biol. Chem.
2005, 280:22925-22936), Parkinson's (Burton et al., Neurotoxicology 2006),
stroke (Satoh et al.,
Proc. Natl. Acad. Sci. U S A 2006, 103:768-773; Shih et al., J. Neurosci.
2005, 25:10321-10335;
Zhao et al., Neurosci. Lett. 2006, 393:108-112), and emphysema (Ishii et al.,
J. Immunol. 2005,
175:6968-6975). We propose that activation of the antioxidant response
element, mediated
through the targeted disruption of the Keap 1 containing ubiquitination
complex or the interaction
between Keapl and Nrf2 may be the long sought after antioxidant therapy for
the various
diseases caused or exacerbated by oxidative stress. However, the development
of safe and
effective small molecule activators of the Keapl-Nrf2-ARE pathway remains a
challenge. Thus,
there is a need for a method for identifying modulators of the Keapl-Nrf2-ARE
pathway, in
particular modulators that inhibit binding of Nrf2 to the Keapl protein and
thus, activate
protective phase 2 oxidative enzymes.
BRIEF SUMMARY OF THE INVENTION
The present invention provides a method for identifying modulators of the
(Keapl
protein, nuclear factor erythroid 2, antioxidant response element pathway
(Keapl-Nrf2-ARE
pathway). In particular, provided is an assay that identifies molecules that
inhibit the binding of
a labeled Nrf2 peptide with the kelch domain of the Keapl protein. Molecules
that inhibit
binding can be activators of the Keapl-Nrf2-ARE pathway. Activation of the
Keapl-Nrf2-ARE
pathway results in an increased accumulation of Nrf2 and the subsequent
induction of protective
enzymes, for example, the phase 2 detoxification enzymes. Activators of the
Keap 1-Nrf2-ARE
pathway are useful for combating oxidative stress-related disorders, such as
those associated with
cancer, emphysema, Huntington's disease, light-induced retinal damage, and
stroke.
Therefore, in one aspect, a method is provided for identifying an agent that
activates the Keap 1-Nrf2-ARE pathway, which comprises providing a mixture
including a Keap 1
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protein or Keap 1-kelch domain polypeptide and an Nrf2 peptide that is capable
of binding the
kelch domain of the Keapl protein; adding an analyte to be evaluated for its
ability to activate the
Nrf2 system to the mixture; and, determining the amount of Nrf2 peptide bound
to the Keapl
protein or Keapl-kelch domain polypeptide, wherein a decrease in the amount of
the Nrf2
peptide bound to the Keap 1 protein or Keap 1-kelch domain polypeptide
compared to the amount
of the Nrf2 peptide bound to the Keap 1 protein or Keap 1-kelch domain
polypeptide in the
absence of the analyte indicates that the analyte activates the Keap 1-Nrf2-
ARE pathway.
In particular aspects of the method, the Nrf2 peptide is radiolabeled or is
labeled
with one member of a donor-acceptor fluorophore pair and the Keap 1 protein or
Keap 1-kelch
domain polypeptide is labeled with the other member of the fluorophore pair
and fluorescence
resonance energy transfer (FRET) or time-resolved FRET (TR-FRET) is measured
to determine
the amount of Nrf2 peptide bound to the Keapl protein or kelch domain wherein
a decrease in
fluorescence over time from the acceptor fluorophore in the presence of the
analyte and/or an
increase in fluorescence over time from the donor fluorophore in the presence
of the analyte
indicates that the analyte activates the Keapl-Nrf2-ARE pathway. In currently
preferred aspects,
the Nrf2 peptide comprises an amino acid sequence selected from the group
consisting of SEQ
ID NO:1 and SEQ ID NO:2 and in further still aspects, the Keap 1 protein or
Keap 1-kelch domain
polypeptide is a fusion protein.
In further aspects, Nrf2 peptide is replaced with an Nrfl peptide or a Pgam5
peptide. In currently preferred aspects, the Nrfl peptide comprises the amino
acid sequence of
SEQ ID NO:7 and the Pgam5 peptide comprises the amino acid sequence of SEQ ID
NO:8.
The above method can be performed using a heterogeneous format wherein the
Keap 1 protein or Keap 1 -kelch domain polypeptide is immobilized to the
surface of a solid
support. In particular aspects, the Keapl protein or kelch domain has a
polyhistidine tag. In
further aspects, the Keap 1 protein or Keap 1-kelch domain polypeptide having
the polyhistidine
tag is immobilized to the surface of the solid support via divalent metal ions
or the Keap 1 protein
or Keapl-kelch domain polypeptide having the polyhistidine tag is immobilized
to the surface of
the solid support using antibodies specific for the polyhistidine tag which
have been immobilized
to the surface of the solid support.
The above method can also be performed using a homogeneous format wherein
the Keapl protein or Keapl-kelch domain polypeptide is labeled with one member
of a donor-
acceptor fluorophore pair and the Nrf2 peptide labeled with the other member
of the fluorophore
pair and fluorescence resonance energy transfer (FRET) or time-resolved FRET
(TR-FRET) is
measured to determine the amount of Nrf2 peptide bound to the Keapl protein or
kelch domain
wherein a decrease in fluorescence over time from the acceptor fluorophore in
the presence of the
analyte and/or an increase in fluorescence over time from the donor
fluorophore in the presence
of the analyte indicates that the analyte activates the Keapl-Nrf2-ARE
pathway.
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Further provided is a method or system for identifying an analyte that is an
activator of the Keapl-Nrf2-ARE pathway, which comprises providing a first
assay wherein a
mixture of a Keapl protein or Keapl-kelch domain polypeptide having a
polyhistidine tag bound
to a labeled Nrf2 peptide is immobilized to the surface of a first solid
support via divalent metal
ions and a second assay wherein a detectable protein having a polyhistidine
tag is inunobilized to
the surface of a second solid support via divalent metal ions; adding an
analyte to be evaluated
for ability to activate the Nrf2 system to the first assay and the second
assay; and determining the
amount of the Nrf2 peptide bound to the Keapl protein or Keapl-kelch domain
polypeptide in
the first assay in the presence of the analyte and the amount of detectable
protein immobilized to
the second solid support in the presence of the analyte, wherein a decrease in
the amount of the
Nrf2 peptide bound to the Keapl protein or Keapl-kelch domain polypeptide
compared to the
amount bound to the Keap 1 protein or Keap l-kelch domain polypeptide in the
absence of the
analyte and no detectable change in the amount of detectable protein
immobilized to the surface
of the second support indicates that the analyte is an activator of the Keapl-
Nrf2-ARE pathway.
In a further aspect, the method includes a third assay in which the Keapl
protein
or Keap 1-kelch domain polypeptide having the polyhistidine tag bound to the
labeled Nrf2
peptide is immobilized to the surface of a third solid support using
antibodies specific for the
polyhistidine tag wherein the antibodies have been immobilized to the surface
of the third solid
support; adding the analyte to be evaluated for ability to activate the Keapl-
Nrf2-ARE pathway
to the third assay; and determining the amount of the Nrf2 peptide bound to
the Keapl protein or
Keapl-kelch domain polypeptide in the third assay in the presence of the
analyte, wherein a
decrease in the amount of the Nrf2 peptide bound to the Keap 1 protein or
Keapl-kelch domain
polypeptide compared to the amount bound to the Keap 1 protein or Keap 1-kelch
domain
polypeptide in the absence of the analyte in the first and third assays and no
detectable change in
the amount of detectable protein immobilized to the surface of the second
support indicates that
the analyte is an activator of the Keapl-Nrf2-ARE pathway.
In particular aspects of the method, the Nrf2 peptide is radiolabeled or is
labeled
with one member of a donor-acceptor fluorophore pair and the Keap 1 protein or
Keap 1-kelch
domain polypeptide is labeled with the other member of the fluorophore pair
and fluorescence
resonance energy transfer (FRET) or time-resolved FRET (TR-FRET) is measured
to determine
the amount of Nrf2 peptide bound to the Keapl protein or kelch domain wherein
a decrease in
fluorescence over time from the acceptor fluorophore in the presence of the
analyte and/or an
increase in fluorescence over time from the donor fluorophore in the presence
of the analyte
indicates that the analyte activates the Keap1-Nrf2-ARE pathway. In currently
preferred aspects,
the Nrf2 peptide comprises an amino acid sequence selected from the group
consisting of SEQ
ID NO:l and SEQ ID NO:2 and in further still aspects, the Keapl protein or
Keapl-kelch domain
polypeptide is a fusion protein.
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In further aspects, Nrf2 peptide is replaced with an Nrfl peptide or a Pgam5
peptide. In currently preferred aspects, the Nrfl peptide comprises the amino
acid sequence of
SEQ ID NO:7 and the Pgam5 peptide comprises the amino acid sequence of SEQ ID
NO:8.
In further aspects, the detectable protein is labeled with a fluorophore or
has a
detectable activity, for example, green fluorescence protein.
Further still is provided a homogenous method for identifying an agent that
activates the Keap 1-Nrf2-ARE pathway, which comprises providing a mixture
that includes a
Keapl protein or Keapl-kelch domain polypeptide labeled with one member of a
donor-acceptor
fluorophore pair and a Nrf2 peptide that is capable of binding the Keapl
protein or Keapl-kelch
domain polypeptide labeled with the other member of the donor-acceptor pair,
wherein the
acceptor fluorophore produces a detectable fluorescence when the Keap 1
protein or Keap 1 -kelch
domain polypeptide is bound to the Nrf2 peptide; adding an analyte to be
evaluated for its ability
to activate the Keapl-Nrf2-ARE pathway to the mixture; and measuring the
amount of the
detectable fluorescence from the acceptor fluorophore over time wherein a
decrease in the
amount of detectable fluorescence over time from the acceptor fluorophore in
the presence of the
analyte indicates that the analyte activates the Keap1-Nrf2-ARE pathway. In
particular aspects,
the donor fluorophore produces a second detectable fluorescence, which
increases over time in
the presence of an analyte when the analyte is an activator of the Keapl-Nrf2-
ARE pathway. In
currently preferred aspects, the Nrf2 peptide comprises an amino acid sequence
selected from the
group consisting of SEQ ID NO:1 and SEQ ID NO:2 and in further still aspects,
the Keap 1
protein or Keapl-kelch domain polypeptide is a fusion protein.
In further aspects, Nrf2 peptide is replaced with an Nrfl peptide or a Pgam5
peptide. In currently preferred aspects, the Nrfl peptide comprises the amino
acid sequence of
SEQ ID NO:7 and the Pgam5 peptide comprises the amino acid sequence of SEQ ID
NO:8.
In further aspects, the donor fluorophore includes a lanthanide and time-
resolved
FRET (TR-FRET) is measured to determine the amount of Nrf2 peptide bound to
the Keapl -
protein or kelch domain wherein a decrease in fluorescence over time from the
acceptor
fluorophore in the presence of the analyte and/or an increase in fluorescence
over time from the
donor fluorophore in the presence of the analyte indicates that the analyte
activates the Keap1-
Nrf2-ARE pathway. In particular aspects, lanthanide is Eu3+ or the donor
fluorophore is
Europium cryptate.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure lA shows a dose response curve for an ion-based assay in which a
mixture
containing His-tagged Keapl-kelch domain polypeptide and 3H-labeled Peptide 2
was incubated
with analyte A. The His-tagged Keapl-kelch domain polypeptide was immobilized
to the
surface of wells coated with divalent nickel ions.
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Figure 1B shows a dose response curve for an ion-based assay in which a
mixture
containing His-tagged Keapl-kelch domain polypeptide and 3H-labeled Peptide 2
was incubated
with unlabeled peptide 2. The His-tagged Keapl-kelch domain polypeptide was
immobilized to
the surface of wells coated with divalent nickel ions.
Figure 2 shows a dose response curve for an antibody-based counterscreen assay
in which a mixture containing His-tagged Keapl-kelch domain polypeptide and 3H-
labeled
Peptide 2 was incubated with various analytes, including analyte A and
unlabeled peptide 2. The
His-tagged Keapl-kelch domain polypeptide was immobilized to the wells of a
plate coated with
anti-mouse IgG using mouse-derived anti-His tag antibodies. =- Analyte A; ^-
Analyte B;
Ananlyte C; 0- unlabeled Peptide 2.
Figure 3 Figure 2 shows a His-GFP binding assay in which His-tagged GFP,
which had been immobilized to the surface of wells coated with divalent nickel
ions, was
incubated with various analytes, including analyte A and unlabeled peptide 2.
The His-tagged
Keapl-kelch domain polypeptide was immobilized to the wells of a plate coated
with anti-mouse
IgG using mouse-derived anti-His tag antibodies. =- Analyte A; ^- Analyte B; 1-
Analyte C; A
- unlabeled Peptide 2.
DETAILED DESCRIPTION OF THE INVENTION
The present invention provides a method for identifying analytes that disrupt
the
interaction of Nrf2 transcription factor with the kelch domain of the Keap 1
protein to form an
Nrf2-Keapl complex. The method provides an assay in which a mixture comprising
a peptide
substrate corresponding to the portion of the Nrf2 transcription factor that
binds to the kelch
domain of the Keap 1 protein and either the Keap1 protein or a polypeptide
comprising the kelch
domain of the Keapl protein (Keapl-kelch domain polypeptide) is incubated with
an analyte
being tested for its ability to disrupt binding of the peptide to the kelch
domain. Analytes
identified using the assay may mimic intracellular disruption of the Nrf2-
Keapl complex, which
leads to increased accumulation of Nrf2 transcription factor and the
subsequent induction of
protective enzymes, for example, the phase 2 detoxification enzymes. Analytes
identified in the
assay as inhibitors of the formation of the Nrf2-Keap 1 complex and thus
activators of the Nrf2
system are useful for combating oxidative stress-related disorders associated
with cancers,
emphysemia, Huntington's disease, light-induced retinal damage, cardiovascular
disease,
Parkinson's disease, Alzheimer's disease, and stroke.
There are many protein:protein interactions required for ARE-mediated gene
induction to occur, such as Nrf2 with small Maf proteins and Nrf2 association
with kinases.
While any of these points of interaction could be potential sites of
modulation of the Keapl-Nrf2
system, detailed structural analysis of the Cu13:Keapl and Keapl:Nrf2
interactions exist and
warrant consideration as targets for small molecule disruptors of these
interactions. Molecules
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that act by disruption of protein:protein interactions in the Keapl-Nrf2
system will provide a
novel means to modulate the system without the liabilities associated with
many of the known
compounds that activate this pathway.
Keap 1:Nrfl Interaction
The Keapl:Nrf2 interface is another site for targeted disruption using small
molecules. High resolution structural information has been gathered for the
Keapl kelch domain
(Li et al., J. Biol. Chem. 2004, 279:54750-54758) and its interaction with
small portions of Nrf2
(Lo et al., EMBO J 2006, 25:3605-3617; Padmanabhan et al., Mol. Cell. 2006,
21:689-700).
These data coupled with mutagenesis data provide a wealth of information on
specific molecular
contacts between Keapl and Nrf2. This information may be exploited by computer
aided drug
design to identify compound that can be tested for the ability to dissociate
Nrf2 and Keapl in
order to enhance the transcription of ARE containing genes.
The three dimensional structure of the Keapl kelch domain was determined by X-
ray crystallography by two different groups (Li et al., J. Biol. Chem. 2004,
279:54750-54758; Lo
et al., EMBO J 2006, 25:3605-3617; Padmanabhan et al., Mol. Cell. 2006, 21:689-
700). The
kelch domain is a six-bladed 0-propeller with each blade consisting of four 0-
sheets and
corresponding to a single kelch repeat motif. The central core of the
structure contains a water
filled channel. The structure determined for the human kelch domain consisted
solely of the
kelch domain (Li et al., J. Biol. Chem. 2004, 279:54750-54758), whereas that
determined for the
mouse contains both the kelch domain and the C-terminal region of 15 amino
acids
Padmanabhan et al., Mol. Cell. 2006, 21:689-700). As expected, the human and
mouse
structures are nearly identical with the exception that the mouse structure
reveals that the C-
terminal region contributes to the folding of the kelch domain. The
significance of this
contribution is underscored by mutagenesis data indicating the critical role
of specific amino
acids in the C-terminal region in enabling Keapl repression of Nrf2
(Padmanabhan et al., Mol.
Cell. 2006, 21:689-700).
The understanding of the molecular interactions between Keapl and Nrf2
increased greatly with the determination of the three dimensional structure of
the mouse Keapl
kelch domain bound to Nrf2 derived peptides of either 9 residues (aa 76-84) or
16 residues (aa 74
to 89) (Padmanabhan et al., Mol. Cell. 2006, 21:689-700) and of the human
Keapl kelch domain
bound to a 16 residue Nrf2 derived peptide (aa 69 to 84) (Lo et al., EMBO J
2006, 25:3605-
3617). The Nrf2 derived peptides adopt a tight type 1(3-turn when bound to
Keapl, with the
highly conserved sequence DxETGE (corresponding to Nrf2 amino acids 77-82)
comprising the
tip of the hairpin (32; Padmanabhan et al., Mol. Cell. 2006, 21:689-700).
Inter-molecular
contacts are made between the side chains of Nrf2 amino acid Glu-79 and Keap 1
amino acid
Ser508, Arg415, and Arg483 (Lo et al., EMBO J 2006, 25:3605-3617; Padmanabhan
et al., Mol.
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Cell. 2006, 21:689-700). A second set of interactions occurs between the side
chain of Nrf2
G1u82 and Keapl Arg380 and Ser363. There are additional contacts made between
the peptide
backbone of Nrf2 and Keap 1 residues. Most of the contacts that occur in the
crystal structure
have been further tested by mutagenesis to establish whether they occur in the
complex
environment of the cell (32). As expected, there is a loss of Nrf2 repression
when the key
contacts in Keapl, Arg380, Arg415 and Arg483, are individually mutated to Ala
(Lo et al.,
EMBO J 2006, 25:3605-3617). Additional Keapl amino acid residues that contact
Nrf2 and
when mutated have a diminished ability to repress Nrf2 include Asn380, Asn382
and Tyr334 (Lo
et al., EMBO J 2006, 25:3605-3617). There are anumber of molecular contacts,
Keapl residues
Ser363, Ser508, G1n530, Ser555, and Ser602 that appear to be somewhat
dispensable, in that
there is a lack of a readily discernable defect in Nrf2 repression when these
residues are mutated
(Lo et al., EMBO J 2006, 25:3605-3617). The structure of the Nrf2 peptide
which inserts into
the Keapl substrate binding site is stabilized by intramolecular interactions
(Lo et al., EMBO J
2006, 25:3605-3617; Padmanabhan et al., Mol. Cell. 2006, 21:689-700).
Phosphorylation of
Nrf2 residue Thr80 or mutation of this amino acid to either Asp or Glu,
results in an apparent
destabilization of the 0-turn and reduced affinity for Keap 1(Lo et al., EMBO
J 2006, 25:3605-
3617). Phosphorylation of Nrf2 at Thr80 and other key residues is a likely
mechanism for the
modulation of Nrf2 activity by stress responsive kinases. Taken together, the
three dimensional
crystal structure coupled with mutagenesis studies has provided a detailed
understanding of the
Keapl:Nrf2 interaction that has the potential to be exploited by drug
discovery efforts.
The stoichiometry of Keapl and Nrf2 remains unresolved, with conflicting data
from two different groups. Keapl is known to form a homodimer through the BTB
domain;
therefore the crystal structure of the kelch domain alone complexed with an
Nrf2 peptide sheds
little light on the stoichiometry. The Yamamoto lab has proposed that the
Keapl dimer binds to
a single Nrf2 molecule (Tong et al., Mol. Cell. Biol. 2006, 26:2887-2900). The
2:1
stoichiometry proposed by Yamamoto and colleagues is based primarily on a
combination of
NMR spectra, isothermal calorimetry and biochemical data. In their model, the
Keapl substrate
binding pocket from one subunit is occupied by the Nhe2 domain ETGE motif,
whereas, the
substrate binding pocket of the second subunit of the dimer is occupied by the
DLG motif, also
located in the Neh2 domain. The binding of a single Nrf2 molecule to a Keap 1
dimer is
proposed to effectively position an alpha-helix containing Lys residues for
ubiquitination (Tong
et al., Mol. Cell. Biol. 2006, 26:2887-2900; McMahon et al., J. Biol. Chem.
2006, 281:24756-
24768). The affinity of the ETGE motif for Keapl is approximately two orders
of magnitude
higher than that for the DLG motif (Tong et al., Mol. Cell. Biol. 2006,
26:2887-2900; McMahon
et al., J. Biol. Chem. 2006, 281:24756-24768). Therefore, it should be
feasible to identify small
molecules that disrupt the low affinity interaction between the Keapl
substrate binding pocket
and the DLG motif. The disruption of the low affinity interaction between Keap
l and Nrf2 is
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unlikely to result in dissociation of Nrf2 from Keap1, due to the high
affinity interaction with the
ETGE motif. However, disruption of the low affinity interaction would be
predicted to block the
ubiquitination of Nrf2 and therefore may lead to activation of the system.
In contrast, a model with 1:1 Keapl:Nrf2 stoichiometry has been proposed by
Hannink and colleagues (Lo et al., EMBO J 2006, 25:3605-3617). This model is
based upon
biochemical and molecular biology experiments. Utilizing differentially tagged
Nrf2 and Keapl,
this group designed an elegant experiment to examine the association of Keapl
and Nrf2 in the
cellular environment. The Neh2 domain of Nrf2 was tagged with either a Heme-
Agglutinin
(HA) tag or a Gal-4 tag, with the two versions being co-expressed in either
the presence or
absence of Keapl . In the absence of Keapl, immunoprecipitation with antibody
to the HA tag
did not pull down Gal-4 tagged Neh2. In contrast, when Keap 1 was co-expressed
in the cell the
Gal-4 tagged Neh2 domain was immunoprecipitated with antibody to the HA tag,
providing
evidence that Keapl linked the two different versions of the Neh2 domain due
to the ability of a
Keapl dimer to bind two molecules of Nrf2 (Lo et al., EMBO J 2006, 25:3605-
3617).
Additional three dimensional structural data will help resolve whether the
Keapl homodimer
binds a single Nrf2 molecule or whether the alternative hypothesis of a 1:1
stoichiometry is
correct, in which case the low affinity interaction between the DLG motif and
Keapl may be a
cryptic function unveiled by the nature of the experiments conducted.
Modulation of Nrf2's activity through disruption of Keap 1 protein:protein
interactions may have advantages over present means of activating the ARE
pathway. Several
known small molecule activators of the ARE system transducer their effects
through electrophilic
attack on the available thiol residues of Keapl, a concept originally proposed
and demonstrated
by Dinkova-Kostova et al. (Dinkova-Kostova et al., Proc. Natl. Acad. Sci. U S
A 2001, 98:3404-
3409). Recent proteomic studies utilizing mass spectroscopy have revealed that
alkylation of
certain cysteine residues contained within the Keapl BACK domain by compounds
such as N-
iodoacetyl-N-biotinylhexylenediamine (IAB) result in Nrf2 translocation to the
nucleus and
ARE-mediated gene expression. This is not a general phenomenon, however, as
not all
electrophiles examined regulated ARE activity. 1-biotinamido-4-(4'-
[maleimidoethyl-
cyclohexane]-carboxamido)butane (BMCC) treatment resulted in alkylation of
Keapl at sites
outside of the linker region, and had no impact on ARE-dependent gene
expression (Hong et al.,
J. Biol. Chem. 2005, 280:31768=3177). Thus, the site of adduction and
resulting ARE induction,
if any, appears highly dependent on the chemistry of the electrophile used as
well as its site of
interaction within Keapl. Sulforaphane, an isothiocyanate and prototypical
inducer of the ARE,
targets cysteine residues within the BTB and kelch domains along with the BACK
domain (Hong
et al., J. Biol. Chem. 2005, 280:31768-3177). Incubation of cells with
sulforaphane also results
in the production of a high molecular weight Keap l complex, which was
subsequently identified
as polyubiquitinated Keapl. From this observation, it has been proposed that
sulforaphane may
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not exert its effects by modulating the physical interaction between Keapl and
Nrf2, but rather
by enabling a transition from Keapl-mediated Nrf2 ubiquitination to Keapl's
autoubiquitination
and subsequent degradation (Zhang and Hannink, Mol Cell Biol 2003, 23:8137-
8151; Zhang et
al., Mol. Cell. Biol. 2004, 24:10941-10953; Hong et al., J. Biol. Chem. 2005,
280:31768-3177).
These electrophiles serve as useful biological tools to explore the mechanisms
regulating ARE-
dependent gene expression and may be clinically useful in acute disease
paradigms. Their
clinical utility in chronic disease, such as neurodegenerative diseases, may
be limited due to
safety concerns arising from their nonspecific alkylation of cellular
proteins.
Modulation of E3 ligase protein:protein interactions
Recently, successful efforts to modulate protein degradation by the ubiquitin-
proteosome pathway have focused on disruption of the protein:protein
interactions within the
ubiquitin conjugating complex or between the complex and its substrate. Using
high-throughput
screening methods, scientists at Roche were able to identify the nutlin class
of compounds and
demonstrate, through crystallographic analysis, that the compounds bound the
p53 binding region
of the mouse double minute 2 (Mdm2) protein, a RING type E3 ubiquitin ligase.
This
competitive inhibition at the p53 site results in the induced expression of
the p21 gene, cell cycle
arrest in the p53 containing cell lines examined, and enhanced cytotoxicity to
p53 containing
cells. In rodent studies, these small molecules reduce tumor volume,
indicating that the
disruption of the E3 ligase-transcription factor complex results in meaningful
in vivo functional
outcomes (Vassilev et al., Science 2004, 303:844-848). The MI-17 (Ding et al.,
J. Am. Chem.
Soc. 2005, 127:10130-1013 1) series and second generation spiro-oxindoles
(Ding et al., J. Med.
Chem. 2006, 49:3432-3435), the HL198 related series (Yang et al., Cancer Cell
2005, 7:547-
559), and Reactivation of p53 and Induction of Tumor cell Apoptosis (RITA;(
Issaeva et al., Nat.
Med. 2004, 10:1321-1328)) were also shown to disrupt the interaction between
Hdm2/Mdm2
and p53. All of these molecules bind to the hydrophobic p53 binding site of
H/Mdm2 (Vassilev
et al., Science 2004, 303:844-848; Ding et al., J. Am. Chem. Soc. 2005,
127:10130-1013 1; Ding
et al., J. Med. Chem. 2006, 49:3432-3435; Yang et al., Cancer Cell 2005, 7:547-
559), except
RITA which binds directly to p53 (Issaeva et al., Nat. Med. 2004, 10:1321-
1328). In many ways,
the Keapl-Nrf2 system is analogous to the H/Mdm2-p53 system. Both
transcription factors, p53
and Nrf2, are targeted for proteosomal degradation by their respective E3
ubiquitin ligases,
Mdm2 and Keapl. Further, Mdm2 is a component of a Cu14A-DDB1 complex (Banks et
al.,
Cell Cycle 2006, 5:1719-1729). Analogous to p53's interaction with H/Mdm2, the
Nrf2 binding
site located on the Keap l kelch domain is a defined pocket on the surface of
the protein
(Klebanoff, J. Leukoc. Biol. 2005, 77:598-625)). The successful identification
of the Mdm2-p53
disrupting compounds provides precedence for the idea that small molecule
protein:protein
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disruptors of the Keap1-containing ubiquitination complex and it's
interactions with Nrf2 will be
realized.
Evidence has emerged over the last decade indicating that activation of the
antioxidant response element may be beneficial to the whole organism. Nrf2
mediated activation
and resultant ARE-regulated gene induction results in improved outcomes in
several animal
models of disease, including (Iida et al., Cancer Res. 2004, 64:6424-6431;
Ramos-Gomez et al.,
Proc. Natl.Acad. Sci. U S A 2001, 98:3410-3415; Xu et al., Cancer Res. 2006,
66:8293-8296;
Yates et al., Cancer Res. 2006, 66:2488-2494), Huntington's (Shih et al., J.
Biol. Chem. 2005,
280:22925-22936), Parkinson's (Burton et al., Neurotoxicology 2006), stroke
(Satoh et al., Proc.
Natl. Acad. Sci. U S A 2006, 103:768-773; Shih et al., J. Neurosci. 2005,
25:10321-10335; Zhao
et al., Neurosci. Lett. 2006, 393:108-112), and emphysema (Ishii et al., J.
Immunol. 2005,
175:6968-6975). We propose that activation of the antioxidant response
element, mediated
through the targeted disruption of the Keapl containing ubiquitination complex
or the interaction
between Keapl and Nrf2 may be the long sought after antioxidant therapy for
the various
diseases caused or exacerbated by oxidative stress. However, the development
of safe and
effective small molecule activators of the Keapl-Nrf2-ARE pathway remains a
challenge. The
present invention provides a method for identifying analytes that disrupt the
interaction of Nrf2
transcription factor with the kelch domain of the Keap 1 protein to form an
Nrf2-Keapl complex
and thus provides a method for identifying activators of the Keap 1-Nrf2-ARE
pathway.
In general, there are two classes of assay formats for methods that can be
used for
identifying inhibitors of Nrf2 binding to the kelch domain of the Keapl
protein depending upon
whether the assay requires the separation of bound species from unbound
species. In assays that
have a heterogeneous format, a separation or isolation step is required to
remove bound material
from unbound material. In contrast, in assays that have a homogeneous format,
removal of
bound species from unbound species is unnecessary. Because homogeneous assays
lack a
separation step, and are more easily automated, they can be more desirable
than heterogeneous
assays in applications that entail the screening of large numbers of analytes.
The method for
identifying analytes that disrupt the interaction of Nrf2 transcription factor
with the kelch domain
of the Keapl protein includes assays that have a heterogeneous format and
assays that have a
homogeneous format. For particular assays, the Keapl-kelch domain polypeptide
comprises
polypeptide in which the kelch domain is fused to a heterologous protein or
polypeptide.
Heterogeneous Format Assays
For assays having a heterogeneous format, the Keap 1 protein or the Keap l-
kelch
domain polypeptide is immobilized onto the surface of a solid support. The
immobilized protein
or polypeptide is then incubated with a mixture comprising a labeled Nrf2
peptide comprising the
amino acid sequence of the Nrf2 transcription factor that binds the kelch
domain of the Keapl
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protein. In currently preferred embodiments, the Nrf2 peptide comprises at
least the 14 amino
acids shown in SEQ ID NOs:I and 2. The analyte being tested for ability to
interfere with the
binding of the labeled Nrf2 peptide to the kelch domain is added to the
mixture at the same time
the labeled peptide is added to the mixture or at a time after the labeled
peptide had been added
to the mixture. Afterwards, the mixture is removed and the amount of labeled
Nrf2 peptide
remaining bound to the Keap 1 protein or Keap 1-kelch domain polypeptide is
determined. If the
analyte disrupts the binding of the Nrf2 peptide to the kelch domain, the
amount of labeled Nrf2
peptide bound to the kelch domain is diminished compared to the negative
control. Preferably,
the assay includes a negative control that does not include the analyte and a
positive control that
includes a molecule that competes with the labeled Nrf2 peptide for binding to
the kelch domain.
A molecule suitable as a positive control is the Nrf2 peptide not labeled.
In a general format for a heterogeneous assay, the Keap1 protein or Keap1-
kelch
domain polypeptide is immobilized to the surface of the solid support by
providing a Keap 1
protein or Keapl-kelch domain polypeptide labeled with a polyhistidine
sequence (His-tag) at the
amino or carboxy terminus of the protein. The His-tagged protein or
polypeptide is then
immobilized on a divalent metal ion solid support, in general, the divalent
metal ion is usually
nickel or copper. Example 4 provides an example wherein a His-tagged Keapl
protein was
immobilized in the wells of multiwell assay plates which had been coated with
divalent nickel
ions. Alternatively, the His-tagged Keapl protein or Keapl-kelch domain
polypeptide can be
immobilized on the solid support using antibodies specific for the His tag.
For example, the
solid support can be coated with anti-mouse IgG antibodies which bind a mouse
anti-His tag
antibody bound to the His-tagged Keapl protein or Keapl-kelch domain
polypeptide. Example 5
provides an example wherein the His-tagged Keapl protein was bound to a mouse
anti-His tag
antibody which was in turn immobilized to the surface of the wells of plates
which had been
coated with anti-mouse IgG antibodies. Alternative means for immobilizing the
Keapl protein
or Keapl-kelch domain polypeptide to the surface of the solid support are well
known in the art
and, include but are not limited to, using antibodies that bind directly to
the Keap 1 protein or
Keap 1-kelch domain polypeptide and which have been immobilized to the surface
of the solid
support, providing the Keapl protein or Keapl-kelch domain polypeptide tagged
with
glutathione-S-transferase (GST) and immobilizing the fusion protein to a solid
support coated
with glutathione, labeling the keap 1 protein or keap 1-kelch domain
polypeptide with biotin and
immobilizing the labeled Keap 1 protein or Keap 1-kelch domain polypeptide to
a solid support
coated with streptavidin, and covalently linking the Keap 1 protein or Keap 1-
kelch domain
polypeptide directly to the surface of the solid support. In a currently
preferred embodiment, the
Keapl-kelch domain polypeptide comprises amino acid residues 322 to 609 of the
human Keapl
protein. In particular embodiments, the Keapl protein or Keapl-kelch domain
polypeptide can
be fused to a heterologous protein or polypeptide.
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In the general format, the Nrf2 peptide substrate is labeled with a detectable
label,
for example, a radiolabel (for example, tritium), a fluorescent label, an
antibody, biotin,
lanthanide ion complex, or the like. The amount of labeled peptide dissociated
from the Keapl
protein or Keapl-kelch domain polypeptide immobilized on the solid support in
the presence of
an analyte is then determined. In particular formats, the Keap 1 protein or
Keap 1-kelch domain
polypeptide is labeled with a detectable label that is distinguishable from
the label on the Nrf2
peptide substrate. In further formats, fluorescence resonance energy transfer
(FRET) is used to
measure the ability of an analyte to interfere with the binding of the labeled
Nrf2 peptide
substrate from immobilized Keap 1 protein or Keap 1-kelch domain polypeptide.
In a FRET
format, the Nrf2 peptide substrate is labeled with a donor fluorophore and the
immobilized
Keapl protein or Keapl-kelch domain polypeptide is labeled with an acceptor
fluorophore or
vice versa. The amount of labeled peptide dissociated from the Keapl protein
or Keapl-kelch
domain polypeptide immobilized on the solid support in the presence of an
analyte is then
determined by measuring the decrease in fluorescence from the acceptor
fluorophore over time in
the presence of the analyte. In some cases, there is also an increase in
fluorescence from the
donor fluorophore. In further formats, FRET is combined with time-resolved
fluorescence (TR-
FRET) or variations thereof. For high throughput screening, it is desirable
that the Nrf2 peptide
be labeled with a fluorescent label or that the assay is performed using a
FRET or TR-FRET
format or variations thereof.
The above format in which a His-tagged Keapl or kelch polypeptide is
immobilized to a solid support via divalent metal ions (ion-based assay) is
desirable because of
the ability to immobilize a greater number of His-tagged protein or
polypeptide per unit area of
the solid support than using antibodies to immobilize the His-tagged protein
or polypeptide
(antibody-based assay). However, as shown in the Examples, analytes can either
compete with
the His-tagged Keapl or Keapl-kelch domain polypeptide for binding to divalent
metal ions or
destabilize the binding of the His-tagged Keap 1 protein or Keap 1-kelch
domain polypeptide to
the divalent metal ions.
The ion-based assay measures the amount of labeled Nrf2 peptide associated
with
the solid support (that is, bound to the kelch domain of the Keap l protein or
Keap l-kelch domain
polypeptide) in the presence of an analyte and any decrease in the amount of
labeled Nrf2 peptide
associated with the solid support during the course of the assay indicates
that the analyte is a
competitor of the labeled peptide for binding to the kelch domain. However,
analytes that
compete with or destabilize the binding of the His-tagged Keapl protein or
Keapl-kelch domain
polypeptide to the divalent metal ions on the surface of the solid support and
not binding of the
Nrf2 peptide to the kelch domain will also cause a decrease in the amount of
labeled peptide
associated with the solid support and thus, a "false positive" result. To
distinguish analytes that
interfere with binding of the labeled Nrf2 peptide with the Keapl protein or
Keapl-kelch domain
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polypeptide from analytes that produce a false positive result, it is
preferable that the assay be
performed using the ion-based assay followed by performing an antibody-based
assay in which a
His-tagged Keapl or Keapl-kelch domain polypeptide is immobilized to a solid
support via
antibodies specific for the His tag. As shown in Example 5, analyte A, which
had appeared to
displace the labeled Nrf2 peptide bound to His-tagged Keapl immobilized to
nickel ion coated
plates as shown in Figure lA was found using the antibody-based assay to
provide inconsistent
positive results (Figure 2) suggesting that the displacement observed in
Figure lA might be the
result of analyte A competing with the His-tagged Keapl protein for binding to
the nickel ion
and not the result of the analyte competing with the labeled Nrf2 peptide for
binding to the kelch
domain of the Keap 1 protein.
While the antibody-based assay can in many cases enable identification of
analytes that interfere with binding of the Nrf2 peptide to the kelch domain
of the Keapl protein
of Keapl-kelch domain polypeptide, the amount of Keapl protein of Keapl-kelch
domain
polypeptide immobilized to a unit area of the solid support is less than that
which can be
immobilized to a solid support via divalent metal ions. Furthermore, for
particular analytes the
results can be inconsistent or equivocal (for example, analyte A as shown in
Figure 2).
Therefore, a counterscreen to the ion-based assay designed to detect analytes
that compete with
the His-tagged protein or polypeptide for binding to the divalent metal ion
attached to the surface
of the solid support was developed that measures the ability of an analyte to
displace the binding
of a His-tagged protein or polypeptide to a divalent metal ion.
In this assay, a His-tagged protein or polypeptide is immobilized to the
surface of
a solid support coated with a divalent metal ion. The immobilized His-tagged
protein or
polypeptide is incubated with the analyte and the amount of His-tagged protein
or polypeptide
dissociated from the divalent metal ions is measured. An increase in the
amount of His-tagged
protein or polypeptide displaced from the solid support indicates that the
analyte is a competitor
of His tag for binding to the divalent metal ion and that the displacement
effect observed in the
ion-based assay is the result of the analyte competing with the His-tagged
Keap 1 or Keap 1-kelch
domain polypeptide with the divalent metal ion and not the analyte competing
with the labeled
Nrf2 peptide for binding to the kelch domain of the Keap 1 protein or the Keap
1-kelch domain
polypeptide. In general, the His-tagged protein can be any protein that is
labeled or has an
activity that can be measured. Example 6 provides an example where His-tagged
green
fluorescence protein (GFP) was immobilized to the surface of multiwell assay
plates coated with
divalent nickel ions and incubated with various analytes, including analyte A.
As shown in
Figure 3, analyte A was able to compete with His-tagged GFP for binding to the
divalent nickel
ions. The results show that the above counterscreen is important for
determining whether the
analytes that had been identified in the ion-based assay interfere with
binding of the labeled Nrf2
peptide substrate with the kelch domain of the Keapl protein or Keapl-kelch
domain
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polypeptide, which is desired, or interfere with binding of the Keap1 protein
or Keap1-kelch
domain polypeptide with the divalent metal ions.
While the ion-based assay can be used to identify analytes that interfere with
or
disrupt binding of Nrf2 to the kelch domain of the Keapl protein, it is
preferable that the method
for identifying such analytes include performing the ion-based assay and
either the counterscreen
assay (for example, the GFP counterscreen disclosed herein) or the antibody-
based assay. In
further still aspects of the method for identifying such analytes, the method
includes performing
the ion-based assay, the counterscreen assay (for example, the GFP
counterscreen disclosed
herein), and the antibody-based assay. Analytes identified using any
combination of
heterogeneous format assays may be useful for use in treatments and therapies
for oxidative
stress-related disorders in an individual where an increase in the
accumulation of Nrf2
transcription factor in the cells of the individual effects the subsequent
induction of protective
enzymes.
Homogeneous Format Assays
A homogeneous format assay can also be used to identify analytes that
interfere
with or disrupt binding of Nrf2 to the kelch domain of the Keapl protein. In
general, a FRET or
time-resolved FRET (TR-FRET) format or variations thereof is used for
identifying analytes that
interfere with or disrupt binding of Nrf2 to the kelch domain of the Keap 1
protein and; therefore,
may be useful in treatments and therapies for oxidative stress-related
disorders. A homogenous
format assay is particularly suitable for high throughput screening assays.
In a FRET format, the Keap 1 protein or Keap 1-kelch domain polypeptide is
labeled with a fluorphore acceptor and the Nrf2 peptide is labeled with a
fluorophore donor or
vice versa. The labeled protein or polypeptide and Nrf2 peptide are incubated
together for a time
sufficient for the Nrf2 protein to bind the kelch domain. When the labeled
protein or polypeptide
and the Nrf2 peptide are bound, the energy transfer between the donor and
acceptor fluorophores
can be measured as fluorescence from the acceptor fluorophore. In some cases,
there is a
decrease in fluorescence from the donor fluorophore. Next, an analyte to be
tested is added and
the affect on the energy transfer between the donor and acceptor fluorophores
is measured. A
decrease in fluorescence from the acceptor fluorophore over time indicates
that the analyte
competes with the labeled Nrf2 peptide for binding to the kelch domain. In
some cases there is
also an increase in fluorescence from the donor fluorophore which can be
measured. Thus, an
increase in donor fluorophore fluorescence indicates that the analyte competes
with the labeled
Nrf2 peptide for binding to the kelch domain. In currently preferred
embodiments, the Nrf2
peptide comprises at least the 14 amino acids shown in SEQ ID NOs:l and 2. In
a currently
preferred embodiment , the Keapl protein or Keapl-kelch domain polypeptide
comprises amino
acid residues 322 to 609 of the human Keapl protein. In particular
embodiments, the Keapl
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protein or Keap 1-kelch domain polypeptide can be fused to a heterologous
protein or
polypeptide.
In an TR-FRET format, the Keapl protein or Keapl-kelch domain polypeptide is
labeled with a fluorophore donor, which comprises a lanthanide preferably in
complex with a
moiety for harvesting light and transferring it to the lanthanide (for
example, a chelate or
cryptate) and the Nrf2 peptide is labeled with a fluorphore acceptor that is
capable of accepting
the energy transfer from the lanthanide or vice versa. The labeled protein or
polypeptide and
Nrf2 peptide are incubated together for a time sufficient for the Nrf2 protein
to bind the kelch
domain. When the labeled protein or polypeptide and the Nrf2 peptide are
bound, the energy
transfer between the donor and acceptor fluorophores can be measured as
fluorescence from the
acceptor fluorophore. Next, an analyte to be tested is added and the energy
transfer between the
donor and acceptor fluorophores is measured. A decrease in fluorescence from
the acceptor
fluorophore over time indicates that the analyte competes with the labeled
Nrf2 peptide for
binding to the kelch domain.
A useful TR-FRET format is called (homogenous time resolved fluorescence or
HTRF, a registered trademark of Cisbio International) wherein the lanthanide
is Eu3+ conjugated
to cryptate (tri sbypyri dine). In an example of an HTRF format, the Keapl
protein or Keapl-
kelch domain polypeptide is labeled with Europium cryptate (trisbypyridine in
which an Eu3+
ion is embedded) and the Nrf2 peptide is labeled with a fluorphore acceptor
that is capable of
accepting the energy transfer from the Europium cryptate (for example, XL665,
a
phycobilliprotein from red algae or vice versa). The labeled protein or
polypeptide and Nrf2
peptide are incubated together for a time sufficient for the Nrf2 protein to
bind the kelch domain.
When the labeled protein or polypeptide and the Nrf2 peptide are bound, the
energy transfer
between the donor and acceptor fluorophores can be measured at 655 nm. Next,
an analyte to be
tested is added and the energy transfer between the donor and acceptor
fluorophores is measured.
A decrease in fluorescence at 655 nm over time indicates that the analyte
competes with the
labeled Nrf2 peptide for binding to the kelch domain and may be useful in
treatments and
therapies for oxidative stress-related disorders.
FRET has been described in, for example, Wolf et al., Proc. Nat. Acad. Sci.
USA
85: 8790-94 (1988) and FRET and TR-FRET have been described in for example
U.S. Patent
Nos. 4,927,923; 5,220,012; 5,432,101; 5,457,185; 5,534,622; 5,346,996;
5,162,508; 5,512,493;
5,627,074; 5,527,684; 5,998,146; and, 6,291,201. Reagents useful for FRET, TR-
FRET, HTRF
are commercially available from vendors such as Cisbio International, Bedford,
MA; Photon
Technology International, Birmingham, NJ; Invitrogen, La Jolla, CA, GE
Healthcare,
Piscataway, NJ.
In any of the aforementioned aspects and embodiments of either the
heterogeneous or homogeneous assay, the Keap 1 protein or Keap 1-kelch domain
polypeptide can
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have an amino acid sequence from any species; however, it is generally
preferred that the Keap 1
protein or Keap1-kelch domain polypeptide have the amino acid sequence of the
human KEAP1.
The amino acid sequence for the human Keapl protein is available from GenBank
under
accession number NP_987096 and NP_036421. In a currently preferred embodiment,
the Keapl-
kelch domain polypeptide comprises amino acid residues 322 to 609 of the human
Keapl
protein. In particular embodiments, the Keapl protein or Keapl-kelch domain
polypeptide can
be fused to a heterologous protein or polypeptide, for example, the
glutathione-S-transferase
(GST), maltose binding protein, thioredoxin, green fluoresecent protein,
biotin carboxyl carrier
protein, c-myc, FLAG, polyhistidine, or the like.
In currently preferred embodiments, the Nrf2 peptide comprises at least the 14
amino acids shown in SEQ ID NOs:I and 2. The Nrf2 peptide comprising the amino
acid
sequence QLDEETGEFLPIQ (SEQ ID NO:2) has a binding affinity for the kelch
domain of
about 130 +/-41 nM. In particular embodiments, the Nrf2 peptide can be
replaced with a peptide
comprising the amino acid sequence of Nrfl or Pgam5, which is capable of
binding the kelch
domain of the Keapl protein (Zhang et al., 2006, J. Biochem. 399:373-85; Lo et
al., 2006, J.
Biol. Chem. Epub ahead of print). The Nrfl peptide can comprise the amino acid
sequence
LLVDGETGESFPAQ (SEQ ID NO:7) and the Pgam5 peptide can comprise the amino acid
sequence RKRNVESGEEELAS (SEQ ID NO:8). The Nrfl and Pgam5 peptides have a
binding
affinity for the kelch domain of about 397+/-133 nM and 626+/-197 nM,
respectively.
The following examples are intended to promote a further understanding of the
present invention.
EXAMPLE 1
The synthesis of Nrf2 Peptide 1 having amino acid sequence LQLDEETGEF(2-
I)LPIQ-OH (SEQ ID NO: 1) was carried out by solid phase peptide methodology.
Nrf2. Peptide 1 was synthesized on an ABI 433A peptide synthesizer (ABI,
Foster
City, CA, USA) using the manufacturer's 0.25 mmol Fastmoc double coupling
protocol with
HATU (O-(7-azabenzotriazole-l-yl)-1,1,3,3-tetramethyluronium
hexafluorophosphate) as the
coupling reagent in 1-methyl-2- pyrrolidinone. The synthesis started with H-
Gln(Trt)-CI-Trt
resin (EMD Novabiochem, EMD Biosciences, San Diego, CA). The protected amino
acids were
Fmoc-Ile, Fmoc- Pro, Fmoc-Leu, Fmoc- Phe(2-I) (2-iodophenylalanine), Fmoc-
Glu(OtBu),
Fmoc-Gly, Fmoc- Thr(tBu), Fmoc- Asp(OtBu), and Fmoc-Gln(Trt). Following
assembly, the
peptide was cleaved from the resin with 95% TFA, 2.5% water and 2.5%
triisopropylsilane for
three hours at room temperature. Following filtration, the filtrate containing
the product was
evaporated to dryness under reduced pressure at room temperature. The residue
was triturated
with anhydrous diethyl ether, filtered, and dissolved in 50%
acetonitrile/water. This peptide
solution was then lyophilized. The crude peptide was purified by RP-HPLC on a
DELTAPAK
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C18 100 A column (Waters, Milford, MA, USA) using a linear A/B gradient.
Solution A was
0.1 % NH4HCO3 in water, and solution B was acetonitrile. Pure fractions, as
determined by
analytical RP-HPLC, were pooled and lyophilized. The correct mass of the
peptide was
confirmed by electrospray mass spectrometry. Amino acid analysis was performed
to confirm
peptide composition and a peptide content value.
To introduce the tritium label, Nrf2 Peptide 1 was stirred with catalysts 10%
Pd/C
and 5% Pd/CaCO3 and tritium gas in DMF on a Tritium manifold for one hour. The
reaction
mixture was filtered and co-evaporated with ethanol in order to remove any
exchangeable
tritium. The crude product was purified by using a semi-preparative HPLC
colunm (Synergy 4u,
Fusion RP 80, 250 x 10 mm column, water containing 0.1% TFA: acetonitrile 75:
25, flow rate 4
mL/min, UV = 254 nm,) to yield [3H] Peptide (7.25 mCi, SA = 17.6 Ci/mmol as
determined by
LC/MS, 25 % yield).
EXAMPLE 2
The synthesis of Nrf2 Peptide 2 having amino acid sequence NH2-
LQLDEETGEFLPIQ-OH (SEQ ID NO:2) was prepared as described for Nrf2 peptide 1
with the
exception that protected amino acid Fmoc-Phe(2-I) was replaced with protected
amino acid
Fmoc-Phe.
EXAMPLE 3
Cloning and purification of recombinant human Keapl-kelch domain by PCR
amplification was as follows.
The DNA encoding the Keapl-kelch domain was PCR amplified using a DNA
template encoding the full-length Keap I. DNAs encoding the full-length Keap 1
protein and
Keapl-kelch domain polypeptide were amplified by PCR using as the template a
DNA clone
encoding the full-length Keapl protein, which had been synthesized by PCR
using overlapping
oligonucleotides based on the published sequence (See for example, Zhang and
Hannink, 2003,
Mol. Cell. Biol. 23: 8137-8151). The human Keapl nucleotide sequence is also
available at
GenBank NM_203500. The primers for amplifying DNA encoding the full-length
Keapl protein
were 5'KFL-Ndel, 5'- GGGcatatgA TGCAGCCAGA TCCCAGG-3' (SEQ ID NO:3) and 3'
KFL BamHI, 5'-CCGggatccT CAACAGGTAC AG-3' (SEQ ID NO:4). The DNA encoding the
Keapl-kelch domain was amplified using primers 5'KK-Ndel-2, 5'-ATGCCCTGCC
GcatatgGCG CCCAAGGTG-3' (SEQ ID NO:5) and 3' KK BamHI-new, 5'-CGggatccGG
TGACAGCCAC GCCCAC -3' (SEQ ID NO:6). The PCR conditions were as follows:
initial
denaturation at 94 C for 10 minutes followed by 35 cycles of 94 C for
30seconds, 62 C for 30
seconds, 72 C for one minute with a final extension of five minutes at 72 C.
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The amplified PCR DNA products were then cloned in pET15b vector (Catalog
No. 70755-3, EMD Novabiochem, EMD Biosciences, San Diego, CA) between the Ndel
and
BamHl sites, to produce plasmid pET 15b-Keap 1-kelch domain. The cloned Keap 1-
kelch
domain polypeptide was in frame with nucleotide codons encoding six
histidines, which
produced a recombinant Keapl-kelch domain polypeptide with a His tag at the N
terminus of the
Keapl-keltch domain. The authenticity of the nucleotide sequence encoding the
recombinant
Keapl-kelch domain polypeptide was confirmed by DNA sequencing. A cDNA
encoding the
full-length Keapl proteins, which had been provided by Mark Hannink at the
University of
Missouri at Columbia (Zhang and Hannink, 2003, Mol. Cell. Biol. 23: 8137-
8151), was used to
provide full-length Keap 1 protein for binding assays.
To overexpress the recombinant Keapl-kelch domain polypeptide, Rosetta2 BL21
PLysS cells (Cat. No. 200131, Stratagene, la Jolla, CA) were transformed with
the pET15b-
Keapl-kelch domain using plates that have 34 g/mL chloramphenicol and 100
g/mL
ampicillin. All colonies were scraped into 10 to 20 milliliters of growth
media. This stock
culture was diluted into multiple flasks containing 1 liter each of growth
media (NH4C1, 1 g;
KH2PO4, 1 g; K2HPO4, 3 g; Na2SO4, 0.3 g; MgC12, 0.05 g; CaC12, 0.005 g, per
liter) and 0.3%
glucose supplemented with chloramphenicol (34ug/mL) and Ampicillin (200ug/mL))
so that the
OD600 nanometers is at or below 0.05. The culture was grown at 37 C with
vigorous shaking
until it reached an OD600 of 0.4 to 0.5. The cultures were cooled down to 25 C
by placing the
flask on ice. The plasmid construct was induced for expression by IPTG
(isopropyl O-D-1-
thiogalactopyranoside) to a final concentration of 1 mM and gently shaking for
24 hours at 15 C,
1 mM PMSF (phenylmethylsulfonyl fluoride) was added simultaneously with the
IPTG to reduce
proteolysis of the expressed protein. The cultures were incubated at 25 C for
another five hours
and cells were harvested by centrifugation at 5000 xg for 15 minutes in a
centrifuge.
The cell pellet was resuspended in 10 mL of lysis buffer (200 mM Tris-HCl pH
8.0, 500 mM sodium chloride, 10 mM Imidazole) containing fresh 1 mM PMSF and 2
mM 0-
mercaptoethanol and protease inhibitor cocktail mix without EDTA. The cell
pellet was frozen
at -80 C and subsequently thawed at 4 C twice.
The DNA present in the lysate was sheared by sonication on crushed ice and the
supernatant fraction was separated from cell debris by centrifugation at
30,000 xg. The
supematant fraction was FPLC purified by passing through a HITRAP Q HP 5 mL
ion exchange
column (GE Health Sciences, Inc., formerly Amersham, Piscataway, NJ). The
bound protein
was eluted with an imidazole gradient to 200 mM Tris-HC1 pH 8.0 buffer
containing 500 mM
sodium chloride and 500 mM imidazole. The major fractions containing
recombinant Keapl-
kelch domain were collected and dialyzed against 50 mM Tris-HCI, pH 8.0
containing 5 mM
DTT. This protein was of sufficient purity to use in binding assays.
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For crystallography, the major fractions containing recombinant Keapl-kelch
domain polypeptide were collected and passed through a MONO Q anion exchange
column
(MONO Q is a trademark of GE Healthcare, Inc.) for further purification. The
protein bound to
the column was eluted with a 1M sodium chloride gradient (IM sodium chloride,
20 mM Tris-
HC1 pH 7.5, 5 mM DTT). The fractions corresponding to protein peaks containing
recombinant
Keapl-kelch domain polypeptide were collected and desalted by dialysis against
20 mM Tris-
HC1 pH 7.5 and 5 mM DTT. Subsequently, the pooled protein fractions were
concentrated using
Amicon concentrators and stored in aliquots at -80 C in PBS containing 20%
glycerol.
EXAMPLE 4
This example provides a protocol for an ion-based assay for identifying
activators
of the Nrf2-Keap 1 system wherein a His-tagged Keap 1 protein or Keap 1-kelch
domain
polypeptide is immobilized to the surface of a 384-well plate via nickel
divalent ions.
Stock solutions containing analytes at various concentrations or 500 nL
dimethylsulfoxide (DMSO) and controls are transferred into the wells of a
CHOICECOAT metal
chelate white 384 well plates (Catalog No. NCI5140 Pierce Biotechnology, Inc.,
Rockford, IL).
The analyte volume is 500 nL per well. Controls include 100 M unlabeled
Peptide 2 and 0.5%
DMSO.
An assay solution comprising 100 ng recombinant His-tagged Keapl protein or
Keapl-kelch domain polypeptide and 50 nM tritium labeled Nrf2 Peptide 2 in 50
L of
phosphate-buffered saline (PBS) per well is prepared. About 50 L of the
prepared solution is
dispensed into each well of the 384 well plates. The plates are covered with
foil seals and
incubated at room temperature for two hours.
After the incubation, the plates are cooled by incubating in a 4 C
refrigerator for
one hour. During the last 15 minutes of the 4 C incubation, one L of cold PBS-
T is added to the
Liquid 1 wash bottle of the EMBLA 96/384 plate washer (Molecular Devices
Corporation,
Sunnyvale, CA). The instrument is primed using program 'Al' a minimum of three
times.
Assay plates are removed from the refrigerator and washed using an EMBLA
96/384 plate washer. The EMBLA 96/384 system is programmed to aspirate the
well, add 80uL
of PBS-T (phosphate buffered saline-0.1% Triton-X), aspirate the well, add 80
L of PBS-T, and
aspirate the well.
About 50 L of MICROSCINT scintillation fluid (trademark of PerkinElmer Life
and Analytical Sciences, Inc., Boston, MA) is dispensed into each of the wells
of the assay plate
and the assay plate is covered with a transparent adhesive seal. Counts per
minute are recorded
(Packard BioScience Company) and percent inhibition calculated for each
compound relative to
DMSO (no activity) and a 100 M solution of Peptide-2 (100% inhibition).
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Shown in Figures lA and 1B are typical results, which were obtained using the
above protocol using Keapl-kelch domain polypeptide and an analyte such as
Analyte A (Figure
lA) or the unlabeled Nrf2 Peptide 2 control (Figure 1B). The unlabeled Peptide
2 control
demonstrates the desired result expected for an analyte that competes with the
labeled Peptide 2
for binding to the kelch domain.
The nickel-based assay is dependent on the Keapl protein or Keapl-kelch domain
polypeptide remaining bound to the nickel on the surface of the plate during
the course of the
assay. If an analyte competes with the Keap 1 protein or Keap 1-kelch domain
polypeptide for
binding to the nickel on the assay plate, then the Keapl-kelch domain
polypeptide bound to the
labeled Peptide 2 will dissociate from the assay plate giving a false positive
signal. To
distinguish between analytes that displace the labeled Peptide 2 from the
kelch binding domain
and that compete with the binding of the Keapl protein or Keapl-kelch domain
polypeptide to
the nickel on the plate surface, a second assay in which there is no
dependence on binding to
nickel to immobilize the Keap 1 protein or Keap 1-kelch domain polypeptide (or
Keap 1 protein)
to the surface of the assay plate was developed. In this assay, which is shown
in Example 5, the
results shown for Compound A in Figure 1A was likely caused by Compound A
competing with
Keapl-kelch domain polypeptide for binding to the nickel on the surface of the
plates.
EXAMPLE 5
This example provides a protocol for an antibody-based binding assay for
identifying activators of the Nrf2-Keapl system. Because Keapl protein's or
Keapl-kelch
domain polypeptide's binding to the assay plate surface is not dependent on
divalent metal
cations such as nickel, the antibody-based assay is also useful for
determining whether the
competitive effect observed in an ion-based assay was a result of the analyte
competing with
binding of the labeled Peptide 2 with the Keap1-kelch domain or with the
binding of the Keap1
protein or Keapl-kelch domain polypeptide with divalent cations on the plate.
In general, the
assay is performed as follows. .
One g/mL of mouse-derived anti-His tag antibody (Catalog No. 35370, Quiagen,
Valencia, CA) is added to anti-mouse IgG-coated white plates (Catalog No.
15234, Pierce
Biotechnology, Inc.) and incubated at room temperature for two hours. The
plate is then washed
with four chamber volumes of PBS to remove any excess antibody.
An assay solution containing 500ng Keapl-kelch protein or Keapl-kelch domain
polypeptide and 200nM tritium-labeled Peptide 2 in 100 L of PBS (per well) is
prepared and
100 L of this assay solution is dispensed into each well of the 384 well
plates. Stock solutions
containing analytes at various concentrations or 500 nL DMSO and controls are
transferred into
the wells. Controls include 100 L unlabeled Nrf2 Peptide 2 and 0.5% DMSO. The
plates are
covered and incubated at room temperature for two hours.
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After the incubation, the plates are cooled by incubating in a 4 C
refrigerator for
one hour. During the last 15 minutes of the 4 C incubation, one L of cold PBS-
T is added to the
Liquid 1 wash bottle of the EMBLA 96/384 plate washer (Molecular Devices
Corporation,
Sunnyvale, CA). The instrument is primed using program 'Al' a minimum of three
times.
Assay plates are removed from the refrigerator and washed using program `06'
on
the EMBLA 96/384. Program 06 is set to aspirate the well, add 80uL of PBS-T,
aspirate the
well, add 80 L of PBS-T, and aspirate the well.
About 50 L of MICROSCINT scintillation fluid is dispensed into each of the
wells of the assay plate and the assay plate covered with a transparent
adhesive seal. Counts per
minute are recorded and percent inhibition calculated for each compound
relative to DMSO (no
activity) and a 100 M solution of Peptide-2 (100% inhibition).
The veracity of the antibody-based assay was tested with Analyte A, Analyte B
(an analyte that binds nickel), and Analyte C over a concentration range of
about 1 nm to about
100 M and using the Keapl-kelch domain polypeptide. The results of the assay
are shown in
Figure 2. As shown in Figure 2, Analyte A, which gave a positive result in
Figure lA, gave
positive but inconsistent results in the antibody-based binding assay. This
suggested that in the
nickel-based assay, Analyte A might have been competing with the Keapl-kelch
domain
polypeptide for binding to the nickel on the surface of the plate and not with
the kelch domain for
binding to the labeled Nrf2 Peptide 2. Figure 2 also shows Analytes B and C
did not compete
with the labeled Peptide 2 for binding to the kelch domain. Using unlabeled
Nrf2 Peptide 2 to
compete with the labeled Nrf2 Peptide 2 for binding to the kelch domain shows
the result from
this assay expected for an analyte that competes with binding of the labeled
Peptide 2 for binding
to the kelch domain.
EXAMPLE 6
This example shows a counterscreen to the nickel-based binding assay that is
amenable to high-throughput screening. In this assay, His tagged Green
Fluorescent Protein
(GFP) retention on a divalent metal cation surface is monitored in the
presence of analytes.
Stock solutions containing analytes at various concentrations or 500 nL DMSO
and controls are transferred into the wells of a CHOICECOAT metal chelate
white 384 well
plates (Catalog No. NCI5140, Pierce Biotechnology, Inc.). The volume is 500 nL
per well.
Controls include 100 .M unlabeled Peptide 2 and 0.5% DMSO.
An assay solution containing one g His tagged GFP protein in PBS (per well)
is
prepared and 50 .L of this assay solution is dispensed into each well of the
384 well plates. The
plates are covered and incubated at room temperature for two hours. Following
the room
temperature incubation, the plates are cooled by incubating in a 4 C
refrigerator for one hour.
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The plates are then washed by aspirating the well, adding 80uL of PBS-T,
aspirating the well,
adding 80uL of PBS-T, and aspirating the well.
Fluorescence emission at 535 nanometers when excited at 405 nanometers is
recorded and percent inhibition of binding of the GFP to the nickel on the
surface of the plate is
calculated for each analyte relative to DMSO (no activity), a 100 M solution
of Peptide 2 (0%
inhibition), and Analyte B (100% inhibition).
Figure 3 shows the results of a typical assay using analytes A B, and C and
Peptide 2. .Analytes that displace the His-tagged GFP from the plate are
undesirable and result in
diminished fluorescent readout and increased percent inhibition. Peptide 2
demonstrates the
desired result from this assay (0% inhibitions) and Analyte B demonstrates
100% inhibition.
Analyte A gave a positive result in this assay, indicating that the effect
seen in the nickel binding
assay was a result of its displacing the His-tagged Keapl-kelch domain
polypeptide from the
plate (undesirable) and not from disruption of the interaction between the
kelch domain and
Peptide 2 (desirable). Because peptide 2 displays no competitive activity in
this assay, but
analyte B, which is known to chelate metal and displace the His-tagged GFP,
achieves 100%
inhibition of binding of the His-tagged GFP to the nickel on the surface of
the assay plate,
demonstrates that this assay is a useful final step of the triage process.
EXAMPLE 7
This prophetic example illustrates a method for identifying activators of the
Nrf2-
Keapl system using a homogeneous time resolved fluorometry (HTRF) assay
performed
essentially according to the directions of the manufacturer, Cisbio
International, Bedford, MA.
Briefly, the Keapl protein or Keapl-kelch domain polypeptide is labeled with
Europium cryptate and Nrf2 Peptide 2 is labeled with XL665 according to the
protocol provided
by the manufacturer. Stock solutions containing analytes at various
concentrations or 500 nL
DMSO and controls are transferred into the wells of 384 well plates. The
volume is 500 nL per
well. Controls include 100 M unlabeled Peptide 2 and 0.5% DMSO.
An assay solution comprising 100 ng Keap 1 protein or Keap 1-kelch domain
polypeptide and 50 nM labeled Nrf2 Peptide 2 in 50 L of PBS per well is
prepared. About 50
L of the prepared solution is dispensed into each well of the 384 well plates
and the
fluorescence from the acceptor fluorophore is measured at 655 nm over time.
Analytes that
disrupt binding of Peptide 2 to the Keapl protein or Keap1-kelch domain
polypeptide cause a
decrease in fluorescence emission at 665 nm, which indicates that analyte may
be useful in
treatments and therapies for oxidative stress-related disorders.
While the present invention is described herein with reference to illustrated
embodiments, it should be understood that the invention is not limited hereto.
Those having
ordinary skill in the art and access to the teachings herein will recognize
additional modifications
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and embodiments within the scope thereof. Therefore, the present invention is
limited only by
the claims attached herein.
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